The buck is placed on a vacuum forming machine and a plastic sheet is heated, then sucked down over top.
In the case of a stamp, you'd create a negative of the visor, then press that into a heated sheet of plastic to create the grooves.
Regarding window tint, having installed the visor either into the helmet or preferably into secondary fixture (for the sake of keeping the correct curvature) you work the film from the center outward, applying a small amount of heat. The film is wetted, then applied to the back side of the visor. A soft tool is used to squeeze the water out exposing the adhesive.
Heating the film makes it shrink but will also allow it to be stretched slightly using your tool in order to conform to the ridges created when making the grooves.
Stamping the plastic sheet (it'd be done with a thicker sheet than vacuum forming) makes applying the window tint easier as the back side is then smooth